During the course of chemotherapy of human cancers, variants which are resistant to multiple drugs frequently arise. We have been investigating the genetic and biochemical basis for this multidrug resistance (MDR) of human tumor cells to chemotherapeutic agents. A model system using the cultured KB cell, a human carcinoma cell line, has been developed in which mutant cells selected independently for resistance to high levels of either colchicine, adriamycin or vinblastine have also been found to be cross-resistant to colchicine, adriamycin, vincristine, vinblastine, puromycin and actinomycin-D. Expression of MDR correlates with the presence of a 4.5 kb mRNA encoded by the mdrl gene which was identified and cloned from highly MDR cell lines in which it is amplified. A complete cDNA coding sequence for the mdrl gene product has been obtained. Transfer of DNA from MDR human cells to drug-sensitive mouse cells results in expression of the complete MDR phenotype and is linked to transfer and expression of the human mdrl gene. The human mdrl gene maps to chromosome 7. Drug-resistance in MDR cell lines results from increased energy-dependent efflux of drugs from resistant cells and correlates with increased expression of a 170,000 dalton glycoprotein on the cell surface. Membrane vesicles from MDR cells bind increased amounts of vinblastine compared to drug-sensitive cells and contain a 170,000 dalton protein which can be labeled with a photoaffinity analog of vinblastine. This labeling is blocked by verapamil, which has been shown to reverse the MDR phenotype in cultured cells. The current working hypothesis is that the 170,000 dalton vinblastine binding protein is the product of the mdrl gene which confers drug-resistance as part of an efflux pump mechanism.